Magnetic flux detector correction system

Abstract

 An aircraft heading correction system utilising the horizontal field strength (HFS) of the earth's magnetic field to determine the tangent of the dip angle (18) generally associated with a given field strength. The tangent of the dip angle (18) is then used to generate a signal (23) which corrects the flux valve heading during accelerated flight.

Description

[0001] The present invention relates to a magnetic flux detector system for determining aircraft heading and more specifically, to a correction system that compensates for heading errors introduced by aircraft acceleration.

[0002] Magnetic flux detector systems for determining aircraft heading, commonly called flux valves, are well known in the art. Currently, flux valves provide accurate heading information only during non-accelerated flight. During accelerated flight the pendulous flux valve element is displaced or "hangs-off" the vertical and senses both the earth's horizontal magnetic field and a portion of the earth's vertical magnetic field. Unwanted sensing of the vertical field introduces errors into aircraft heading measurement. Prior art systems simply disengaged the flux valve heading output during periods of accelerated flight that exceeded a predetermined threshold. During periods when the flux valve is disengaged, heading information is obtained from a free gyroscope. As a result, prior art systems have the disadvantage of operating for long periods using only free gyro heading information when aircraft acceleration is above a given threshold. Below the acceleration threshold, flux valve heading errors are still introduced as the aircraft experiences small accelerations.

[0003] The present invention is defined in the appended claims and compensates for flux valve heading errors during periods of low level acceleration thereby increasing flux valve heading accuracy and decreases dependence on obtaining heading information from a free gyro. Magnetic heading and horizontal field strength are derived from a flux valve output. The tangent of the dip angle is determined from the horizontal field strength and combined with the aircraft's magnetic north and east accelerations to produce a correction signal which is then subtracted from the flux valve deviation angle signal to provide a corrected magnetic heading.

[0004] An aircraft heading correction system in accordance with the present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a block diagram of the flux valve correction system of the present invention,

Figure 2 illustrates resolving the Earth's magnetic field into horizontal and vertical components,

Figures 3 and 3a illustrate the flux valve hang off angles, and

Figure 4 illustrates heading error in terms of north/east field strength vectors.

[0005] To simplify the understanding of the present invention, it will be explained by using a block diagram as shown in Figure 1. However, it will be understood that the present invention may be implemented as part of a programmable digital computer.

[0006] Referring now to the flux valve hang-off correction system 10 shown in Figure 1, a flux valve 11 and associated circuitry (not shown) provides signals on lines 12 and 13 to a horizontal field strength computation block 14 and to a flux valve heading block 15.

[0007] The flux valve 11 measures magnetic fields which lie parallel to its sensitive axis and develops a signal representative of a deviation angle Ψ FV from North which is the magnetic heading. The flux valve 11 output may be provided to a programmable digital computer through a current servomechanism and an analogue-to-digital converter. Flux valve 11 and associated circuitry output is comprised of signals on lines 12 and 13 consisting respectively of:-
signal X₁ = field strength x sine (Ψ FV),
signal X₂ = field strength x cosine (Ψ FV),
where Ψ FV = deviation angle.

[0008] The flux valve 11 sensitive element is pendulously suspended. The horizontal field component has a field strength that varies in magnitude according to geographic location near the earth's surface. Since the horizontal field component is always aligned with the magnetic north/south grid line, the flux valve output signals 12 and 13, respectively, reduce to:
X₁ = horizontal field strength x sine (magnetic heading)
X₂ = horizontal field strength x cosine magnetic heading)

[0009] Signals representative of the values X₁ and X₂ are provided to horizontal field strength block 14 and raw flux valve heading block 15. Signals representative of magnetic North and East acceleration of the aircraft are provided to horizontal field strength block 14 and correction term block 16. The output of the horizontal field strength block 14 (HFS) appearing on line 17 may be expressed as a signal having the form:-
[X₁² + X₂²]^½

[0010] The output of the flux valve heading block 15 may be expressed as a signal having the form:-
Tan⁻¹ [X₁/X₂].

[0011] When the invention is implemented by a programmable digital computer, the value of horizontal field strength (HFS) may be maintained by updating its value during periods of very low aircraft acceleration through the use, for example, of a five minute time constant, single pole filter. A shorter time constant filter, for example 3 minutes, may be used during ground alignment.

[0012] The tangent of the dip angle (i.e. the inclination angle at which the magnetic field enters the earth) may be obtained from a horizontal field strength signal on line 17.

[0013] The tangent of the dip angle signal (TanDip) appearing on line 19 is calculated by the tangent of the dip angle block 18 utilising correlation polynomials which relate horizontal field strength to magnetic dip. The magnetic data utilised to develop these polynomials was obtained from Geological Survey Circular 873 and International Geomagnetic Charts and Grid Values (IAGA Bulletin No. 47).

[0014] Polynomials have been derived for use in calculating the tangent of the dip angle in block 18. Each polynomial corresponds to a different region on the earth's surface. The polynomials take the following form:-
TanDip (represented by the signal on line 19) = A₀ + A₁ x HFS + A₂ x HFS² + A₃ x HFS³
where HFS is the horizontal field strength on line 17 in volts expressed as HFS = Nano Teslas HFS x 216.699 x 10⁻⁶.

[0015] The correction term block 16 receives the signals representing tangent of the dip angle on the line 19 from the block 18 and the aircraft magnetic North/East acceleration on line 21. The form of the correction term Ψ ERR on line 20 will be discussed subsequently.

[0016] Referring now to Figure 2, the earth's magnetic field (B_E) incident to the north-east down coordinate system has corresponding vector components of horizontal field strength (HFS) and vertical field strength (VFS). The angle between the B_E vector and HFS vector is the dip angle γ.

[0017] When an aircraft accelerates the pendulous element of flux valve 11 is forced to "hang-off" to an angle approximately equal to the inverse tangent of the aircraft horizontal acceleration divided by the down acceleration.

[0018] Referring now to Figures 3 and 3A, aircraft accelerations are resolved into horizontal components along the approximate magnetic north (Figure 3) and east (Figure 3A) coordinates through direction cosines. Figures 3 and 3A illustrate the resulting flux valve hang-off angles α and β due to north and east accelerations, respectively. The corresponding signals representative of magnetic north and east field strength measurements may be defined as follows:-
North component = [tan (γ) x sine (α) + cos (α)]
East component = [tan (γ) x sin (β) ]
Where α = tan⁻¹ (A_NM/A_DOWN), β = tan⁻¹ (A_EM/A_DOWN)
Where A_NM = aircraft Magnetic North acceleration,
A_EM = aircraft Magnetic East acceleration and
A_DOWN = aircraft Down acceleration.

[0019] Since HFS is defined to be north, any east component results in a heading error as shown in Figure 4 which is equal to:
Flux Valve Heading Error = Tan⁻¹ [east component/­north component]
or
Ψ ERR = Tan⁻¹ [tan (γ) sin (β)/tan (γ) sin (α) + cos (α) ]

[0020] Referring again to Figure 1, the correction term block 16 provides the correction term signal Ψ ERR on line 20 and signal Ψ ERR is subtracted from the deviation angle Ψ FV in the subtraction block 22 during small aircraft accelerations. The output of the subtraction block 22 on line 23 is the corrected magnetic heading.

Claims

1. An aircraft heading correction system of the type utilizing a magnetic flux detector to provide a signal as a function of magnetic heading of the aircraft, characterised in that the system comprises first means (14) responsive to a signal representative of north, east and down acceleration components of said aircraft, and to said signal as a function of magnetic heading of the aircraft, for providing a signal representative of horizontal field strength, second means (18) responsive to the signal representative of horizontal field strength, for providing a signal representative of the tangent of a dip angle, third means (13) responsive to said signal as a function of magnetic heading of the aircraft, for providing a magnetic heading signal, fourth means (16) responsive to the signal representative of horizontal field strength, the signal representative of the tangent of the dip angle, and the signal representative of north, east and down acceleration components of the aircraft, for providing a magnetic heading correction signal, and fifth means (22) responsive to the magnetic heading correction signal and the magnetic heading signal for providing a corrected magnetic heading signal.

2. A system according to claim 1, characterised in that said signal as a function of magnetic heading of the aircraft comprises a first and second signal having the relationship:-
first signal = X₁ = horizontal field strength x sine (magnetic heading)
second signal = X₂ = horizontal field strength x cosine (magnetic heading)

3. A system according to claim 2, characterised in that the signal representative of horizontal field strength has the form:-
horizontal field strength = [X₁² + X₂²]^½

4. A system according to claim 2 or 3, characterised in that the third means (15) provides a magnetic heading signal having the form:-
tan⁻¹ [X₁/X₂]

5. A system according to any of the preceding claims, characterised in that the signal representative of the tangent of the Dip angle is of the form:-
tangent of Dip = A₀ + A₁ x HFS + A₂ x HFS² + A₃ x HFS³
where A₀, A₁, A₂ and A₃ are predetermined constants
HFS = the horizontal field strength.

6. A system according to any of the preceding claims, characterised in that the magnetic heading correction signal is of the form:-

where
east component = [tan(dip angle)x sin A_EM/A_DOWN],
north component = [tan(dip angle)x sin (A_NM/A_DOWN) + cos (A_NM/A_DOWN)],
A_EM = aircraft east acceleration,
A_NM = aircraft north acceleration,
A_DOWN = aircraft down acceleration.

Drawing